Prepare the marinade for the chicken. Whisk together 1/2 cup water with 1/2 cup sugar until sugar dissolves. Add remaining ingredients (vinegar, fish sauce, garlic, chili). Add chicken and stir to coat. Marinate for at least 4 hours and up to overnight.

Remove chicken from the marinade, discarding the marinade. Toss chicken in potato starch, making sure to evenly coat all sides. Shake off excess potato starch.

Gently place some of the chicken pieces into the hot oil and fry until golden brown on all sides. Once the chicken is cooked, remove from oil and let drain on a paper-towel lined plate. Repeat with remaining chicken.

Pour half of the sauce on top of the chicken and toss to coat. Top with fresh cilantro and serve with remaining sauce on the side.